More specifically, what are the pros/cons of each? Why do some cars use one system over the others and which is “better”?
EDIT: Thanks everyone for the explanations. It’s been very useful.
In: 10
Fwd/rwd have been well covered. Important to note that 4wd/AWD both have a variety of implementations and there’s enough corner cases and abuse by the marketing team to make you bang your head on a wall.
The other important thing is that all of the wheels of a car turn at different speeds because of their different paths as the vehicle turns. Hence why axles have differentials and for the simplest AWD system there’s also a centre differential to allow the front and rear wheels to turn at different combined speeds. This can be improved by limited slip differentials that only allow a certain difference in speeds between them or various other computer controlled cunningness.
A 4wd on the other hand is designed to function in a low traction environments and so can take advantage of that to have the wheels slip instead.
So they have the front and rear propshafts locked together to give more traction. Additionally a 4wd normally has a selectable low range to give more torque at low speeds (plus allow slow speeds without burning a clutch out or overheating an automatic gearbox). For on road usage of a 4wd they’re either selectable 4wd and disconnect the front axle (making them rwd) or are constant 4wd’s and have a centre diff that’s lockable.
Some obnoxious corner cases are disco 2’s which are 4wd’s complete with solid axles, high/low range but didn’t come factory with a linkage to lock the centre diff, various Subaru’s which had hi/lo in a car, Mitsubishi’s super select system for their 4wd’s that gives you rwd, AWD, locked centre diff/4wd and then hi/low.
There’s also some clever systems that normally only drive one axle but when loss of traction is detected engage the other axle.
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